The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The overall disclosure is related to a new light emitting diode (LED), photoelectron source, and photodetector (i.e., solar cell) of high-performance and the methods of making same. Each of the three types of devices has its own unique design and fabrication, while they share the use of a general architecture of a nanoscale metallic photonic resonant cavity and certain nanofabrication methods. However, it should be pointed out that for each device type, the photonic cavity has been tailored to have special properties and/or functions that other device types do not possess, and the same or similar nanofabrication methods have been used in different forms and/or sequences special to each particular device type. For examples, for LEDs, the photonic cavity is used as antenna for enhancing the radiation of light generated inside of the cavity; while for the photoelectron source and the photodetector, the photonic cavity is used as good light absorber and trapper for enhancing the light transmission from outside of the cavity into inside, the trapping light inside the cavity, and the total light absorption by a semiconductor(s) inside of the cavity. For the photoelectron source, the semiconductor inside the photonic cavity is further configured to have thin thickness and short carrier lifetime to shorten the photoelectron pulse width and to reduce the photoelectron energy and emittance spread (i.e., variation). The particular background for each device is discussed separately below.
(1) Light Emitting Diode (LED)
In one embodiment, this disclosure is related to a new light emitting of high-performance and the methods of making same. Various embodiments of the described invention can solve the challenges in a conventional LEDs, either individually or together, that include how to (a) enhance light radiation in light emitting material, (b) effectively extract the light from the inside of the light-emitting material to outside; (c) replace indium-tin-oxide (ITO) transparent electrode; (d) when pumped by a light source not by an electric current, achieve (i) high light transmission from the outside to inside of the LED and (ii) high light trapping and absorption in a very thin light generating material of the LED to maximize the quantum efficiency (the efficiency of converting the incoming light to the emitted light by the material), and (e) better cooling.
To overcome these issues, the light emitting diode (LED) described herein uses a metallic photonic resonant cavity antenna to greatly enhance the extraction of light from the light-emitting material inside of the cavity to the free space outside of the cavity antenna, and use subwavelength metal mesh to replace ITO.
(2) Photoelectron Source
In another embodiment, this disclosure is related to a new semiconductor photoelectron source of high-performance and the methods of making same. A semiconductor photoelectron source uses a semiconductor to convert incoming photons to electrons, and have the electrons going out of the semiconductor into a free space for a further use. The further use includes electron microscopy, electron lithography, and generation of x-ray using electron beam. Often the electrons in the free space get further extracted, focused and accelerated.
The current disclosed embodiment of this invention is to solve the challenges in a conventional semiconductor photoelectron source, either individually or together, that include how to (a) shorten the photoelectron pulse, (b) reduce photoelectron energy spread, (c) reduce emittance, (d) increase quantum efficiency (the efficiency of converting the incoming photons to photoelectrons. High quantum efficiency can reduce the power of a pumping laser/lamp that provides incoming photons), (e) better cooling, (f) long photoelectron source lifetime, and (g) less laser jitter.
For examples, conventional semiconductor photoelectron source designs, limited by intrinsic principles, have several drawbacks. A conventional semiconductor photoelectron source uses a thick semiconductor layer (or a bulk) for photoelectron emission material, excited by a pulsed laser which is focused and shaped by conventional diffractive-limited optics without any light resonant cavity. The major drawbacks for such approach include (i) long electron pocket length (>10 ps); (ii) large longitudinal emittance; (iii) less than 5% quantum efficiency; (iv) strong image charge effect; (v) poor cooling to GaAs surface and hence limited life-time; (vi) the minimum laser spot size (hence electron beam transverse size), set by the light diffractive limit, is about 1 micron; and (vii) laser beam jitter (due to vibration) which further prevents a use of a small diameter beam line.
To overcome the challenges, some embodiments of the invention described herein uses different physical principles and designs that are paradigm shifts from the conventional approaches. Various embodiments of the photoelectron source as described herein can have seven (7) elements that a conventional photoelectron sources does not have to overcome these challenges. They include one or more of: (1) the photoelectron emission semiconductor (GaAs) layer (less than or equal to about 100 nm) is ultra-thin much less than light absorption length, (2) the photoelectron emission semiconductor layer also has ultra-short carrier lifetime (e.g., less than about 0.2 ps), (3) a resonant cavity (formed by the metallic lens, the GaAs and a metallic backplane) to enhance the light transmission into the GaAs and the absorption of the majority of incoming fs laser pulse in the GaAs (greater than about 80%)—hence greatly improving quantum efficiency, (4) plasmonic subwavelength lens(es) (made of Au) on top of the GaAs to focus and shape the light into a (an array of) hole(s) of subwavelength size, (5) a metallic backplane under the thin GaAs to reduce image charge effect, enhance cooling and light harvesting, (6) an additional voltage applied between the top metal and the back metallic plane to create an additional electrical field inside the GaAs for better electron extraction, and (7) a segmented aperture array to divide incoming light into subwavelength pockets.
(3) Photodetector (Solar Cell)
In another embodiment of the invention as described herein is a new photodetector of high-performance and the methods of making same. The new photodetector also can be a high performance solar cell, and therefore the wording “photodetector” and “solar cell” are interchangeable in the specification.
Some of these embodiments can solve the challenges in a conventional photodetectors, either individually or together, that include how to (a) achieve high light transmission from outside of the photodetector to inside, (b) achieve high light trapping and absorption in a photodetector where a light photodetection material in the photodetector has a thickness only a fraction of the light absorption-length in the photodetection material, (c) replace indium-tin-oxide (ITO) transparent electrode, and (d) provide better cooling. The improvement in (a) and (b) can greatly improve the quantum efficiency (converting photons to electrons) and hence power conversion efficiency, leading to cheaper and more economical viable photodetectors/solar cells. The improvement in (c) can lengthen the photodetector lifetime (i.e., the length of usage) as well as improve photon to electron conversion performances, which is temperature dependent.
As such, the inventors have invented and demonstrated a new ultra-thin high-efficiency photodetector (i.e., solar cell (SC)), termed “plasmonic cavity with subwavelength hole-array (PlaCSH) solar cell”, that offers a solution to all three issues with unprecedented performances. PlaCSH-SC consists of a metallic photonic resonant cavity comprising: a 30 nm thick metal-mesh electrode with subwavelength hole-array (MESH) that replaces ITO, a metal back electrode, and in-between a polymer photovoltaic active layer of 85 nm thick (⅓ average absorption-length). Experimentally, PlaCSH-SCs achieved light coupling-efficiency/absorptance as high as 96%, average 90%, broad-band, and Omni acceptance (light coupling nearly independent of both light incident angle and polarization); and an external quantum efficiency of 69% for only 27% single-pass active layer absorptance; leading to a standard-solar-irradiation power conversion efficiency (PCE) of 52% higher than the reference ITO-SC (identical structure and fabrication to PlaCSH-SC except MESH replaced by ITO). The Omni acceptance of PlaCSH is over 2 fold lower in high angle reflectance than Si subwavelength antireflection, yet PlaCSH has a thickness over 10 fold thinner, and can, in harvesting scattered light, increase PCE by additional 81% over ITO-SC, leading to a total 175% increase. Furthermore, we found that the light reflection and absorption by MESH after formation of PlaCSH are reduced by 2 to 6 fold from the values when it is alone; while the sheet resistance of MESH is 2.2 ohm/sq or less—over 4.5 fold better than ITO film, giving a lowest reflectance-sheet-resistance product. Fabrication of PlaCSH used nanoimprint on 4″ wafer and is scalable to roll-to-roll manufacturing. The designs, fabrications, and findings are applicable to thin solar cells in other materials.